CN1330902C - High flow rate gas delivery - Google Patents

Abstract

The present invention is directed to a method of delivering liquefied compressed gas at a high flow rate, comprising: sending the liquefied and compressed high-purity semiconductor gas into a storage tank; arranging a temperature measuring device on the wall of the compressed gas storage tank; placing at least one heating device adjacent to the storage tank; monitoring the resulting temperature by means of a temperature measuring device; placing a pressure measuring device at the outlet of the storage tank and monitoring the storage tank pressure; adjusting the heat output of the heating device to heat the liquefied compressed gas in the storage tank to control the evaporation of the liquefied compressed gas in the storage tank; and controlling the flow of gas from the storage tank.

Description

High flow rate gas delivery

field of invention

The present invention relates to high flow rate gas delivery, and more particularly, the present invention relates to a method and system for delivering high purity semiconductor gases at high flow rates.

Background of the invention

There is a growing need in semiconductor manufacturing to deliver specialty gases at high flow rates to the point of use. Traditional compressed gas storage tanks, ie cylinders and ton containers, hold liquefied gas at its own vapor pressure at ambient temperature. When the steam is vented from the tank, the liquid evaporates at an equivalent rate, resulting in a drop in pressure. This consumes energy from the remaining liquid in the tank. With no heat transfer to and from the tank, the temperature of the liquid drops, causing a corresponding drop in vapor pressure. Further vapor discharge eventually subcools the liquid and reduces vapor flow.

Along with liquid subcooling, rapid vapor discharge and uncontrolled heat transfer to the storage tank also cause extreme heating of the tank walls. This transfers the metastable droplets into the vapor phase. Alternatively, traditional compressed gas storage sources deliver saturated steam. A drop in its temperature or flow restrictions in the production line cause condensation. Droplets in the vapor stream are detrimental to most instruments and need to be minimized.

Therefore, the problem is to deliver high vapor flow rates from conventional sources with minimal liquid carryover and no liquid subcooling.

The prior art discloses some means of delivering high vapor flow rates from conventional sources, but none of the prior art teaches or suggests a method and system for doing this at high flow rates from an external source while having Optimal heat transfer to the liquid and minimized droplet formation in the production line.

US Patent No. 6,122,931 discloses a system that transfers liquefied gas from a storage tank to a distillation column and uses the distillate to deliver the ultra-high purity vapor to the point of use. An additional processing step is included using liquefied gas.

US Patent No. 6076359 discloses increasing the heat transfer between the environment and a cylinder placed in a gas chamber. This increase is achieved by changing the air flow rate in the air chamber and adding cooling fins inside the air chamber. This enhances heat transfer from the environment to the cylinder. The resulting flow rate is relatively low. However, the increase in delivery flow rate is still not enough to meet the current demand.

US Patent No. 5894742 discloses a liquefied compressed gas pumped into a evaporator which converts the liquid into a vapor phase before delivering the gas to the point of use. Using a plurality of such evaporators, each corresponding to a point of use, allows a high throughput through the delivery system.

US Patent No. 5673562 discloses the use of a storage tank equipped with an internal heat exchanger, which maintains the temperature of the liquid-gas interface. Heat is transferred to the interface by radiation or conduction through the gas phase.

US Patent No. 5644921 discloses superheating steam discharged from a storage tank containing liquefied compressed gas heated using an external heat exchanger. Then the steam is passed through the heating tube immersed in the liquid phase, and the superheated steam is used to exchange heat with the liquid phase. This cools the vapor and causes the liquid to boil to maintain a minimum vapor pressure in the tank. The cooled steam is then transported to the point of use.

All of the methods proposed in the above mentioned patents provide a means of supplying additional energy to the liquid via an external source. However, these methods are not suitable for existing sources of compressed gas storage and require additional equipment. This made those inventions cost a lot of money. Furthermore, these inventions only address the problem of supplying additional energy to the system. There are no discussions or suggestions on ways to achieve optimal operation of the delivery system, reducing various thermal resistances.

Udischas R. et al. "Comparison of performance and cost of various bulk electronic specialty gas delivery schemes" (presented in Workshop on Gas Distribution System, SEMICON West 2000) compares the economic advantages of various delivery systems for compressed gases. The maximum delivery flow rates used for comparison were 400 standard liters per minute (slpm) ammonia flowing for two hours and 1000 slpm HCl flowing for one hour.

"High Flow Delivery Systems for Bulk Specialty Gases" by Yucelen B et al. (Workshop on Gas Distribution Systems, presented at SEMICON West 2000) discloses that externally heated ton vessels are capable of delivering high flow rates (up to 1500 slpm). The focus of this document is the analysis of water carryover in steam at high flow rates.

In view of the prior art, there is a need for a method and system that 1) facilitates venting steam from existing compressed gas storage sources (gas cylinders and ton containers) at high flow rates using an external heat source; 2) provides a controlled strategies to achieve optimal heat transfer from the storage tank wall to the liquid; and 3) develop a method to deliver high vapor flow rates while also minimizing droplet formation in the production line.

Summary of the invention

One aspect of the present invention is directed to a method for controlling the temperature of liquefied compressed gas in a storage tank, comprising: allowing liquefied compressed gas to enter the storage tank; placing a temperature measuring device on the wall of the compressed gas storage tank; placing at least one heating device It is placed near the storage tank; the temperature of the compressed gas in the storage tank is monitored by a temperature measuring device; and the output of the heating device is adjusted to heat the liquefied compressed gas in the storage tank.

In another embodiment, the present invention is directed to a method for maintaining vaporization of liquefied compressed gas in a storage tank during vapor delivery, comprising: feeding liquefied compressed high-purity semiconductor gas into the storage tank; placing a temperature measuring device On the storage tank wall; place at least one heating device near the storage tank; monitor the temperature of the compressed gas in the storage tank through a temperature measuring device; place a pressure measuring device at the outlet of the storage tank; monitor the compression in the storage tank through a pressure measuring device the pressure of the gas; release a portion of the gas from the storage tank; and adjust the heat output of the heating device to maintain the desired pressure.

In another embodiment, the present invention is directed to a method of delivering liquefied compressed gas at a high flow rate, comprising: sending liquefied and compressed high-purity semiconductor gas into a storage tank; placing a temperature measuring device on the wall of the compressed gas storage tank ; placing at least one heating device adjacent to the storage tank; monitoring the resulting temperature by means of a temperature measuring device; placing a pressure measuring device at the outlet of the storage tank and monitoring the storage tank pressure; adjusting the heat output of the heating device so that the temperature in the storage tank The liquefied compressed gas is heated to control the evaporation of the liquefied compressed gas in the storage tank; and the gas flow from the storage tank is controlled.

In another embodiment, the invention is directed to a method for delivering ammonia at a high flow rate comprising: delivering high purity liquefied compressed ammonia gas into a tonnage vessel; placing a thermocouple on the wall of the tonnage vessel; At least one heating device is placed adjacent to the tonnage vessel; monitor thermocouples; place a pressure transducer at the outlet of the tonne vessel and monitor storage tank pressure; monitor the average weight loss of liquefied compressed ammonia in the tonne vessel; temperature to heat liquefied ammonia in tonnage vessels; to boil liquefied compressed ammonia under convective and nucleate boiling conditions; to control the evaporation of liquefied compressed ammonia in tonnage vessels under convective and nucleate boiling conditions; Ammonia flow in ton containers.

The invention is also directed to a system for delivering semiconductor process gas at a high flow rate, comprising: a storage tank containing liquefied compressed semiconductor process gas; a temperature measuring device placed on the wall of the storage tank; a pressure probe placed in the storage tank The outlet; the heating device is placed near the storage tank, where the temperature detector and the pressure detector are used to adjust the output of the heater, so as to heat the liquefied compressed semiconductor gas in the compressed gas storage tank, and realize the semiconductor gas from the compressed gas storage tank a high flow rate of gas; and a valve arrangement controlling the flow rate of the semiconductor gas from the storage tank.

Storage tanks are gas cylinders or ton containers. Liquefied can be ammonia, hydrogen chloride, hydrogen bromide, chlorine or perfluoropropane. Typically, the temperature measuring device is a thermocouple. The heating device is a ceramic heater, a heating mantle or a thermal fluid heat transfer device.

The term "high flow rate" as used herein means in the present invention the velocity at which gas flows out of the storage tank. For purposes of the present invention, the term "high flow rate" refers to a rate exceeding or about 500 slpm.

"Storage tank" as used herein means any container in the present invention that holds liquefied gas. For purposes of this invention, a "storage tank" is a gas cylinder or ton container. Other types of storage tanks capable of storing liquefied gas are also contemplated herein.

As used herein, "adjacent" means to indicate an immediately adjacent position. In at least one embodiment, "near" means a location of the heating device proximate to the storage tank.

Detailed description of the drawings

Through the description of the following preferred embodiments and the accompanying drawings, those skilled in the art will understand other purposes, features and advantages of the present invention, in the figure:

Figure 1 provides a schematic representation of heat transfer on storage tank walls in the present invention;

Figure 2 provides a typical boiling curve of a liquid;

Figure 3 provides a schematic diagram of the experimental setup used to deliver high steam flow rates in the present invention;

Figure 4 provides a schematic diagram of the change in ammonia flow rate and the change in surface temperature over time;

Figure 5 provides a schematic diagram of the delivery system;

Figure 6 provides a flow diagram of the delivery system; and

Figure 7 is a schematic diagram of a prototype ammonia delivery system.

Detailed Description of the Invention

The present invention achieves optimal heat transfer of liquefied gas in storage tanks by limiting liquid boiling in natural convection and nucleate boiling regimes. The present invention provides a transfer heat flux of up to about 180 kWm ^-2 to deliver ammonia up to about 1000 slpm while maintaining the liquid at near ambient temperature. One example shows a transfer heat flux of approximately 93.5 kWm ^-2 for delivery of approximately 500 slpm of ammonia. Similar transfer heat fluxes and flow rates apply to other similar semiconductor gases and are determined by the properties of those gases.

The present invention is capable of delivering high vapor flow rates at relatively low surface temperatures, where the surface temperature is not expected to be more than 20°C above the bulk liquid or ambient temperature. Delivery of high steam flow rates at lower heater temperatures can be accomplished by enhancing heat transfer from the heater to the storage tank.

The present invention allows the full available surface area to be used for heat transfer to the liquid phase by an external heat source, for example using a hydrothermal bath. The control strategy allows achieving and maintaining high vapor flow rates at low surface temperatures, as well as enhanced heat transfer to the liquid. In addition, the system and method of the present invention also makes it possible to reduce droplets in the vapor phase by superheating the vapor phase in the storage tank without any additional equipment. Means for reducing external and internal resistance to heat transfer are also provided.

In the proposed system, a heat source external to the system is used to provide energy to evaporate the liquid. The heat source can be a heating mantle or a thermal fluid in direct contact with the storage tank. In the case of hot fluids (such as water or oil), submerging the storage tank in a bath provides the lowest resistance to heat transfer (see Table 1). In the case of a heating jacket, the heater is designed for a higher temperature in order to compensate for poor contact between the heater and the storage tank. This allows sufficient energy to be transferred to the liquid even if the effectiveness of the heater-storage tank contact deteriorates over time. Frequent changes of compressed air tanks, which are unavoidable at high flow rates, may reduce contact effectiveness. In addition, it is difficult to accurately repeat this contact after each cylinder change. The use of conductive grease or rubber between the heating jacket and the storage tank further reduces external contact resistance.

A control strategy is proposed to minimize the internal resistance to heat transfer at the tank-liquid contact. This strategy limits liquid evaporation in both convective and nucleate boiling regimes. This is accomplished by monitoring the temperature of the tank surfaces in contact with the liquid, as well as the pressure of the tank. A decrease in surface temperature indicates that the heat flux from the heat source to the storage tank is lower than the energy required to vaporize the liquid at a given flow rate. This indicates high external thermal resistance. Increasing the temperature of the heat source (thermal fluid or heating jacket) increases the heat flux in this case. The rise in surface temperature that accompanies the pressure drop indicates that the heat flux from the storage tank to the liquid is lower than the energy required to vaporize the liquid. This indicates the onset of film boiling of the vapor, which increases the internal resistance to heat transfer. Lowering the heater temperature increases the heat flux in this case.

The same heat source is also used to transfer heat to the vapor phase, resulting in the delivery of superheated steam. This minimizes the number of liquid droplets in the gas phase and reduces the use of complex instrumentation in order to prevent condensation of vapors in the production line. The superheated steam provides the energy required to vaporize the liquid droplets present in the vapor phase. Superheated steam also compensates for cooling on flow restrictions that minimize steam condensation.

The present invention does not require the use of new storage tanks and is capable of delivering a wide range of vapor flow rates from conventional compressed gas storage tanks, thereby reducing capital investment and meeting customer needs. The strategy proposed by the present invention controls liquid evaporation in both convective and nucleate boiling regimes, thereby increasing the rate of heat transfer. Increased heat flux over a given temperature results in an optimal method for high flow rate delivery.

The basis of the invention concerns the energy balance around the liquid in the compressed gas container, delivering the vapor at the flow rate F, as shown in equation (1).

${\mathrm{<span\; class="notranslate">\; mCp</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}\frac{{\mathrm{<span\; class="notranslate">\; dT</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}}{\mathrm{<span\; class="notranslate">\; dt</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; F\&Delta;H</span>}}_{\mathrm{<span\; class="notranslate">\; vap</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; sat</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

The energy required for vaporization FΔH _vap (P _sat ) is detectable heat loss (drop in liquid temperature T _L ) or heat transfer (Q) from the heat source. ΔH _vap (P _sat ) is the latent heat of vaporization at the saturation pressure P _sat . The symbol m represents the mass of the liquid and Cp _L is its heat capacity.

To maintain a constant liquid temperature ( _dTL /dt=0), the heat transfer to the liquid must equal the energy required to vaporize at a given flow rate. The heat transfer source may be an external heater or may be the environment. The rate of heat transfer from a heat source depends on the available surface area (A), the bulk heat transfer coefficient (U), and the temperature difference between the source and the liquid (T _o -T _L ), as shown in Equation (2).

Q＝UA(T _o -T _L ) (2)

Figure 1 schematically represents the resistance to heat transfer from a heat source to a liquid through a section through a storage tank wall. In the figure, T _o , Tw _out , Tw _in and T _L represent the temperature of the heat source, the outer wall of the storage tank, the inner wall of the storage tank and the liquid, respectively. The relationship of the overall heat transfer coefficient U to the heat transfer coefficient h _out from the heat source to the storage tank, the thermal conductivity k _W of the storage tank wall, and the heat transfer coefficient h _in from the tank wall to the liquid is shown below.

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; UA</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;r</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}{\mathrm{<span\; class="notranslate">\; Lh</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;r</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}{\mathrm{<span\; class="notranslate">\; Lh</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; In</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; o</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;k</span>}}_{\mathrm{<span\; class="notranslate">\; w</span>}}\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Equation (3) assumes a long cylindrical storage tank of length L with an inner diameter r _i and an outer diameter r _o . For a given storage tank, the thermal resistance due to thermal conductivity (In(r _o /r _i )/(2Δk _w L)) is fixed. However, the internal (1/(2Δr _i Lh _in ) and external (1/(2Δr _o Lh _out ) thermal resistances depend on the operating parameters and the heat source. Table 1 below lists typical thermal resistance values for various conditions (explained later) .

Table 1: Comparison of thermal resistance to heat transfer based on Figure 1

The listed values are calculated for an alloy steel tonnage vessel having a 24 inch outside diameter and a 0.5 inch wall thickness. The length of the heating is assumed to be 5 feet. For these calculations, an ambient temperature of 21°C and a hot water temperature of 27°C were used. For the internal heat transfer resistance, the ammonia characteristic at ambient temperature is used. The relevant method for calculating the heat transfer coefficient is taken from "Heat and Mass Transfer" (Perry's Chemical Engineers Handbook, 7th Edition, Chapter 5, McGraw-Hill, 1999). Table 1 shows that still ambient air provides the greatest thermal resistance to external heat transfer. Using these values in the above equation, it can be shown that the heat transfer from the ambient air is only sufficient to deliver about 7 slpm of ammonia without significant liquid subcooling. This calculation assumes a 10°C drop in liquid temperature from cryogenic cooling. In the absence of an external heat source, heat transfer from the environment limits the rate at which the liquid vaporizes. To obtain high vapor flow rates without liquid subcooling, external heat sources such as heating mantles or hot liquid baths can be used, as are known in the art. Besides, the heating device in the present invention also includes other ceramic heaters or other suitable devices and methods known in the art for providing heat to the storage tank. Using a hot water bath greatly reduces the external thermal resistance shown in Table 1. In the case of a heating jacket, the thermal resistance to heat transfer will depend on the contact between the heating jacket and the storage tank. The presence of an air gap between the heating jacket and the storage tank reduces heat transfer due to the air acting as an insulator. However, a higher heating jacket temperature compensates for poor contact between the heater and the storage tank. Placing a heat transfer fluid between the heater and the storage tank also enhances heat transfer.

As mentioned above, the overall heat transfer coefficient also depends on the internal thermal resistance to heat transfer. The internal heat transfer coefficient depends on the temperature difference between the tank wall and the liquid and the boiling characteristics of the liquid. Generally speaking, the pool boiling characteristics can be roughly divided into four states, as shown in the typical boiling curve in Figure 2. The boiling curve is a log-log plot of heat flux per unit area (Q/A) versus the temperature difference between the tank wall and the liquid (Tw _in -T _L ). The four states are: natural convective boiling, nucleate boiling, vapor film boiling, and radiative boiling. The schematic diagrams on the curves represent the characteristics of each state. In natural convective boiling, the heated liquid near the hot wall is lifted by natural convection and evaporates at the vapor-liquid interface. In the nucleate boiling regime, vapor bubbles form on the tank walls and rise to the liquid-gas interface without condensing in the bulk liquid. As the temperature difference increases, the gas bubbles coalesce and form a vapor film on the wall surface. This is called film boiling. Although the temperature difference in this state is higher, the rate of heat transfer is lower than that of nucleate boiling. At higher temperature differences, the wall temperature rises significantly, resulting in radiative heat transfer. In this state, the heat flux to the liquid increases again as the temperature difference increases. However, this state is of no practical use for heating due to wall temperatures close to the melting point of conventional storage tank materials (in excess of about 1000°C).

Table 1 lists the typical values of the thermal resistance of ammonia under different boiling states. High heat transfer rates are obtained under natural convection and nucleate boiling regimes at extremely low temperature differences ( _Twin -T _L ) due to the low thermal resistance to heat transfer in the liquid phase. In the film boiling regime, the vapor film acts as a thermal insulator. This reduces the heat flux to the liquid due to the higher thermal resistance in the vapor phase. This shows that in order to deliver high vapor flow rates from a compressed gas container, the vaporization rate must be increased by reducing the overall thermal resistance. The energy for vaporization can be provided by an external heat source. Low thermal resistance to internal heat transfer can be obtained by controlling the boiling of liquids in natural convection or nucleate boiling regimes.

Vapor delivered from a traditional compressed air source is saturated because it is in equilibrium with the liquid present in the container. In the process line, the steam can be cooled due to the lower temperature of the line or the temperature drop during the expansion process on the flow restriction. The drop in temperature of saturated steam causes condensation. The presence of droplets may be harmful to the instrument. Providing energy to the vapor to compensate for the cooling effect minimizes the possibility of droplet formation.

example

Experiments were conducted to evaluate the feasibility of using tonnage vessels to deliver high flow rates of ammonia. These experiments were used to establish the relationship between steam flow rate and surface temperature.

The experimental setup used to test the ton-scale container is shown in Fig. 3. A 4130X alloy steel ton vessel filled with -530 lbs of ammonia was used for this experiment. Twelve ceramic heaters are used to control the surface temperature of the tonnage vessel. The heater is a 1" x 1" square grid of ceramic covered with a continuous heating wire. Each ceramic grid measures 6" x 19.5" and is rated at 3.6kW. These heaters are spaced ~1" apart and fit in groups of four longitudinally on the grid. Three of these are placed side by side longitudinally along the vessel. One inch thick insulation is used over the heaters and the entire assembly is secured to the t The bottom of the ton-scale vessel. This heating device covers -25% of the total surface area of the ton-scale vessel.

Surface temperature is monitored and controlled at six different locations using thermocouples arranged in a zigzag pattern across the heated surface area. Thermocouples are fixed to the surface of the tonnage vessel by joint welding to obtain the true surface temperature. Each thermocouple is used to control a bank of heaters using a simple on-off controller. This setup allows for maintaining a consistent temperature on the heating surface. Ammonia flow rate is measured as the average weight loss over a period of 30 to 50 minutes.

Figure 4 illustrates the change in ammonia flow rate (right y-axis) and the change in surface temperature (left y-axis) over time. An increase in surface temperature results in a corresponding increase in the flow rate of ammonia vapor delivered by the tonnage vessel. The pressure and liquid temperature, which were also monitored, were kept constant during this experiment. The increased heat flux at higher surface temperatures results in a higher vaporization rate, which increases the flow rate. The constant pressure and liquid temperature indicated that the energy provided by the heater was sufficient and all energy was used to vaporize the ammonia and maintain the flow rate. The temperature differences observed at various flow rates and actual heat flux to the liquid are shown in Table 2. The heat flux is calculated using the ammonia flow rate and the heat of vaporization.

Table 2: Experimental results

Temperature difference(Tw _out -T _L )(℃)   Average ammonia flow rate (F) (slpm) Average heat flux (Q/A)(kW/m ^-2 ) 2.78 150 28.61 3.33 327 62.25 4.22 363 70.60 5.56 492 93.51

The maximum heat flux that can be transferred to liquid ammonia during nucleate boiling is 1.5×10 ³ kWm ⁻² . This heat flux corresponds to the maximum point on the boiling curve shown in Figure 2 between the bubble nuclei and the vapor film boiling regime. The maximum heat flux was calculated using the correlation method taken from "Heat and Mass Transfer"(Perry's Chemical Engineers Handbook, 7th Edition, Chapter 5, McGraw-Hill, 1999). Using the heat transfer correlation of nucleate boiling, it can be further shown that for ammonia, the temperature difference between the tank wall (Tw _in ) and the bulk liquid (T _L ) at this point of maximum heat flux is °20°C. In the experiments, the observed temperature difference between the storage tank outer surface (Tw _out ) and the bulk liquid (T _L ) was less than 6°C at a flow rate of 492 slpm. This suggests that even at such high flow rates, the liquid is only just beginning to nucleate boil. Higher flow rates are conveniently achieved in this state due to enhanced heat transfer. Conceptually, with the experimental configuration described above, 7890 slpm of ammonia could be delivered while still boiling the liquid ammonia in the nucleated state.

The experimental data presented above demonstrate that with an adequate heat source and controlled surface temperature, it is possible to deliver a range of steam flow rates from tonnage vessels. The data also show that these high flow rates can be achieved right at the onset of nucleate boiling.

Figure 5 illustrates a preferred system. The system includes the following components: 1) traditional compressed gas source (i.e. gas cylinder, ton container); 2) heat source; 3) valve device (i.e. control the gas flow from the gas source); 4) pressure measurement device (i.e. monitor 5) a temperature measuring device (ie, a temperature sensor that measures the temperature in the air source); 6) a heater control box to control heat; and 7) a flow switch to control air flow. In Fig. 5, the solid line in the figure represents the gas flow rate, and the dotted line represents the control loop.

Compressed gas storage tanks are filled with liquefied gas at its own vapor pressure at ambient temperature. This produces a liquid phase at the bottom of the vessel and a vapor phase at the top. The gas to be delivered is discharged from the vapor phase by fully opening the valve. In this configuration, a pressure sensor reads the vapor pressure inside the tank. A temperature sensor is used to monitor the surface temperature at the bottom of the tank, which is always in contact with the internal liquid phase. Monitor the temperature in at least three different locations. The average of these readings is used in the control logic. The temperature sensor can be an infrared sensor or a thermocouple soldered on the joint. The control box can be a computer or a real-time logic controller. A heat source is used to transfer heat to the storage tank.

The heat source can be a jacket heater or a recirculating liquid tank. The temperature of the fluid in the tank is maintained by an external heater, and its flow rate is controlled by a flow meter. The jacket heater can be powered by electricity (as in an experimental setup, a resistive heater) or by recirculating thermal fluid. The lowest thermal resistance to heat transfer is obtained if the storage tank is submerged in a bath of heating fluid such as water or oil and the fluid is recirculated. If a heating jacket (electrical or thermal fluid) is used, it is best to use conductive grease or rubber to increase the contact area between the heater and the storage tank.

The proposed control strategy for the conveying system is schematically shown in Figure 6. The basis for the control decision is to compare the values of pressure P and surface temperature T at the current time t with the values at the previous time t-Δt. This improves the safety of the delivery system by placing an upper limit on the temperature (T _max ) as well as on the pressure (P _max ), and also ensures uninterrupted delivery. A decrease in surface temperature as the flow rate increases is a warning of insufficient heating. If the heater delivers less heat than is needed to evaporate the liquid to maintain the flow rate, the surface temperature will drop. An increase in surface temperature as pressure decreases indicates boiling in the vapor film regime. In this case, the energy transferred from the storage tank to the liquid is less than that required for vaporization.

In no flow conditions (no processing demand for the gas), the valve remains fully open and the pressure and temperature read by the pressure and temperature sensors are constant. The line is filled with gas. When the customer needs gas, the flow switch opens, triggering the control loop. When the gas starts to flow the pressure drops, and depending on the flow rate, the surface temperature remains constant or decreases. This triggers the controller to increase the heater temperature to maintain a constant pressure. In the case of recirculating fluid heaters (tanks or heating jackets), the fluid temperature or fluid flow rate can be effectively used to control the heat supplied to the storage tank. The heat provided by an electric heater is controlled by the voltage applied to the heater or by alternating the heater on and off. The heater provides energy to the liquid phase, causing the liquid to vaporize, which maintains a constant pressure in the storage tank. The pressure drop resulting from further increases in flow rate requirements at constant or falling surface temperatures increases the power to the heater to evaporate more liquid. The increased heat input keeps the liquid at a constant temperature.

As the flow rate requirement decreases, the pressure in the storage tank increases due to the accumulation of steam, which also increases the surface temperature due to the accumulation of heat. When these two conditions are met, the controller lowers the heater temperature. The heater temperature also decreases in the following two cases. First, if an upper pressure or temperature limit is reached. Second, if the surface temperature rises, the pressure drops at the same time (steam film boiling). Before re-entering the control loop each time, check the flow switch to verify that gas is still required.

Heating the entire storage tank also results in heat transfer to the vapor phase in the storage tank. This creates superheated steam in the container. The excess energy of the superheated steam assists in the evaporation of any metastable droplets that may have been carried into the vapor phase. Venting superheated steam from a compressed air source minimizes the chance of steam condensation in the production line, which can be a serious problem when venting saturated steam. Drops in temperature or flow restrictions can cause condensation of droplets in the production line while using saturated steam.

Examples of Bulk Ammonia Delivery Systems

A prototype bulk ammonia delivery system to be installed at a customer site is described below, employing possible variations of the control strategy described above.

The conveyor system consists of ton-scale containers enclosed in and supported by sheet metal enclosures. The ton vessel is a horizontal pressure tank with an internal volume of ~450L and a weight of ~529kg. Ton container shells have internally coated insulation. The shell is divided horizontally near the center line of the ton. The upper half is hinged to provide access for maintenance, installation and dismantling of the tonnage container. The edges are rigid enough not to deform. Three ring supports embedded in the bottom of the enclosure are spaced equidistantly along the ~52" cylindrical section of the ton vessel. The central support has facilities for mounting two infrared detectors for monitoring the surface temperature of the ton vessel. At each end of the enclosure, placed in Slanted rails on the longitudinal centerline are incorporated and connected to the end brackets to facilitate positioning of tonnage containers.

In order to transfer heat to the ton container, four ceramic heaters of 12" inner diameter x 11" wide x 28" circumference are used. The heaters are installed in the shell between two sets of brackets. The ton container The heater arrangement on the cylindrical section is shown in Figure 7. Each heating zone has a maximum operating temperature of 850°F and is rated at ~3.75kW, 440V, single-phase power. Each heating zone is equipped with a fixed flange at each end, suitable for Spring fixed. The spring connects the inside of the housing so that the heater exerts a grip on the tank as the ton is lowered into position. The geometry of the heating band/spring/ton surface ensures that the band heater never supports the ton weight, and provides the best surface contact between the heater and the storage tank. This configuration also allows for changing tonnage vessels without repeated adjustments.

Each heater strip is connected to a process controller and is equipped with two thermocouples. One thermocouple is used to control the temperature set point and the other is used for overheat monitoring. The process controllers for the four heaters are installed in a common control box. The control box applies power to the heater through the power junction box and reads the temperature through the signal junction box.

Enter the pressure setpoint in the universal controller before starting flow. The heater is turned off when the pressure set point is reached and turned back on when the pressure drops below the set point. Pressure will drop due to steam venting. Heater temperature set points are entered into each process controller to set an upper limit for heater temperature. An overheat condition sends a signal through the control box to the common controller, which turns off the heaters. In addition to the overheat signal, the heater process controller will also provide a signal to the general controller in the event of a heater burnout or failure, which will initiate an alarm, shut down power to the heater, and initiate an automatic switchover to backup equipment .

Two infrared surface temperature sensors are connected to the common controller and act as the main safety device by limiting the wall temperature of the tonnage vessel. They have a maximum configurable value of 125°F. Below the configured surface temperature, the sensor allows the heater to operate. If the surface temperature set point is reached on either sensor, this signal overrides the pressure control process described above. The common controller shuts down power to the heaters and initiates an automatic switchover to the backup. A temperature sensor is also installed in the process line for steam temperature measurement and the signal is sent to the general controller. In the present invention, the temperature measuring means is any temperature sensor, preferably a thermocouple. The temperature difference between the steam temperature and the average value of the two infrared sensors is compared with the set point configured in the controller.

The temperature difference above the set point is used as a warning that the liquid is too cold. This provides operators with the opportunity to reduce ammonia requirements or switch to another tonnage vessel.

To summarize the control strategy, the heater is powered on if all of the following conditions are met: 1) tonnage vessel surface temperature is below set point; 2) tonscale vessel vapor pressure is below set point; 3) no heater Overheating; 4) All heaters on; 5) Tonnage vessel surface/steam differential below set point. If any of conditions 1), 3) or 4) are not met, the heater is powered off and the general controller initiates an automatic switchover to the second tonnage container.

Testing of the prototype delivery system demonstrated that an average ammonia flow rate of 600 slpm could be maintained at a delivery pressure of 90 psi (pounds per square inch) for ~2.5 hours without significant liquid subcooling. Tests also demonstrated that, with the configuration described above, a maximum ammonia flow rate of 800 slpm could be delivered for 30 minutes without significant pressure drop in the tonnage vessel.

The semiconductor gas used in the present invention can be any liquefied compressible gas, preferably ammonia, hydrogen chloride, hydrogen bromide, chlorine and perfluoropropane.

While the above described invention is fully functional, certain variations are contemplated. Some modifications may be required to redesign the storage tank.

Storage tanks can be designed as an integral part of the tank wall. This configuration will provide heat transfer coefficients similar to those obtained when using a hot liquid bath.

The use of internals such as fins can be added to the storage tank to increase the heat transfer area. This has the potential to deliver high flow rates at lower temperatures. Heat transfer is enhanced if all available external surfaces of the storage tank are heated and there are highly conductive internal fins extending into the liquid in both the vapor and liquid spaces.

External fins can also be added to enhance heat transfer from the sump to the storage tank.

Thermocouples or thermal wells can be included in the storage tank for direct liquid temperature measurement. This enables more robust control as the liquid temperature rather than the storage tank pressure is held constant.

Molecular sieve pads or other separation unit operations such as distillation on the outlet can be used to reduce impurities such as moisture in the vapor phase to deliver ultra-high purity gas to the point of use.

Bleeding a percentage of the original vapor headspace to the abatement system reduces light component impurities, resulting in delivery of ultra-high purity gas.

The invention can also work in continuous mode. The liquid evaporator can be designed according to the proposed invention. Existing storage containers can be modified to receive liquid product continuously. Liquefied gas is pumped into this evaporator where it is continuously evaporated to deliver the gaseous product to the point of use. Pump rate depends on flow rate requirements. The flow rate requirement and desired steam temperature control the heat flux to the evaporator.

Specific features of the invention are given in one or more of the drawings for convenience only, individual features may be combined with other features in accordance with the invention. Those skilled in the art will know of other implementations which are also within the scope of the claims.

Claims (10)

1. A method for controlling the temperature of liquefied compressed gas in a storage tank, comprising: a. Send the liquefied compressed gas into the storage tank; b. The temperature measuring device is arranged on the inner wall of the compressed gas storage tank; c. placing at least one heating device adjacent to said storage tank; d. monitoring the temperature of the compressed gas in the storage tank through the temperature measuring device; and e. adjusting the output of the heating device to heat the liquefied compressed gas in the storage tank. 2. A method for maintaining evaporation of liquefied compressed gas in a storage tank during vapor delivery, comprising: a. Send high-purity semiconductor liquefied compressed gas into the storage tank; b. setting the temperature measuring device on the inner wall of the storage tank; c. placing at least one heating device adjacent to said storage tank; d. monitoring the temperature of the compressed gas in the storage tank through the temperature measuring device; e. placing a pressure measuring device at the outlet of said storage tank; f. monitoring the pressure of the compressed gas in the storage tank through the pressure measuring device; g. venting a portion of the gas out of the storage tank; and h. Adjusting the heat output of the heating device to maintain the desired pressure. 3. A method for delivering liquefied compressed gas at a high flow rate, comprising: a. Send high-purity semiconductor liquefied compressed gas into the storage tank; b. The temperature measuring device is arranged on the inner wall of the compressed gas storage tank; c. placing at least one heating device adjacent to said storage tank; d. monitoring the temperature generated by said temperature measuring device; e. placing a pressure measuring device at the outlet of the storage tank and monitoring the storage tank pressure; f. adjusting the heat output of the heating device to heat the liquefied compressed gas in the storage tank, thereby controlling the evaporation of the liquefied compressed gas in the storage tank; and g. Controlling the flow of said gas from said storage tank. 4. A method for delivering ammonia at a high flow rate, comprising: a. Send high-purity liquefied compressed ammonia into ton-level containers; b. Thermocouples are arranged on the inner wall of the tonnage container; c. placing at least one heating device adjacent to said tonnage vessel; d. monitoring said thermocouple; e. placing a pressure sensor at the outlet of the tonnage container, and monitoring the pressure of the storage tank; f. monitoring the average weight loss of said liquefied compressed ammonia in said tonnage container; g. adjusting temperature from the output of said heating means to heat said liquefied ammonia in said tonnage vessel; h. boiling said liquefied compressed ammonia under convective and nucleate boiling regimes; i. controlling the evaporation of said liquefied compressed ammonia in said tonnage vessel under said convective and nucleate boiling conditions; and j. Controlling the flow of ammonia from said tonnage vessel. 5. A system for delivering semiconductor process gases at high flow rates, comprising: a. Storage tanks containing liquefied compressed semiconductor production gases; b. The temperature measuring device is arranged on the inner wall of the storage tank; c. a pressure detector placed at the outlet of the storage tank; d. a heating device placed near the storage tank, wherein the temperature measuring device and pressure detector are used to adjust the output of the heater so as to heat the liquefied compressed semiconductor gas in the compressed gas storage tank, and achieving a high flow rate of semiconductor gas from said compressed gas storage tank; and e. Valve means for controlling the flow of said semiconductor gas from said storage tank. 6. The system of claim 5, wherein the storage tank is a gas cylinder or a ton container. 7. The system of claim 5, wherein the heating device is a heating mantle. 8. The system of claim 5, wherein the heating device is a ceramic heater. 9. The system of claim 5, wherein the high flow rate is up to 500 slpm. 10. The system of claim 5, wherein the temperature measuring device is a thermocouple.

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